A conventional system for detecting the rotational speed of a rotating body usually consists of generating a signal which corresponds to the rotational position of the rotating body and processing this signal so as to obtain a rotational speed-detecting signal.
FIGS. 1A and 1B schematically illustrate a device which is usually used for obtaining a rotational position-detecting signal in the system for detecting rotational speed.
In FIG. 1A, a rotary disc 3 is attached to a rotary shaft 2 of a rotating body 1 of which the rotating speed is to be detected and is permitted to rotate together with the rotating body 1. An electromagnetic position detector element 4 is secured at a position adjacent to the peripheral surface of the rotary disc 3 to detect the rotational position. FIG. 1B illustrates the position detector element 4 and the rotary disc 3 on an enlarged scale. As shown in FIG. 1B, a predetermined number N of projections 31 are formed around the circumference of the rotary disc 3 in the form of a gear, the projections 31 being arrayed so as to equally divide the whole circumference of the rotary disc 3. As the rotary disc 3 rotates, the individual projections 31 successively pass a position adjacent to the position detector element 4. Therefore, the distance between the position detector element 4 and the rotary disc 3 alternates, whereby the position detector element 4 produces an output signal 5 having a sinusoidal wave form. The frequency of the sinusoidal wave changes in proportion to the rotational speed of the rotating body 1, but the amplitude remains constant irrespective of the rotational speed.
FIG. 2 illustrates a conventional circuit 6 which is used for converting the output signal 5 of the position detector element 4 into a signal that is proportional to the rotational speed. FIG. 3 shows various voltage wave forms which indicate the operation characteristics of the circuit of FIG. 2.
In FIG. 2, the output signal 5 of the position detector element 4 is supplied to one input terminal of a comparator 61, another input terminal of which is connected to Ov. The output signal 5 of the position detector element 4 assumes a sinusoidal wave form as denoted by V.sub.1 in FIG. 3, and the output signal of the comparator 61 assumes a rectangular wave form as denoted by V.sub.2 in FIG. 3. The output signal of the comparator 61 is supplied to the input terminal of a differentiation filter 62, and the output signal V.sub.3 of the differentiation filter 62 is supplied to a one-shot circuit 63. The output signal V.sub.4 of the one-shot circuit 63 is supplied to a low-pass filter 64 and the output signal V.sub.5 of the low-pass filter 64 is produced as a rotational speed-detecting signal.
The output signal V.sub.3 of the differentiation filter 62 has positive pulses that correspond to the rising portions of the rectangular wave V.sub.2 and negative pulses that correspond to the falling portions of the rectangular wave V.sub.2. Responsive to positive pulses in the output signal V.sub.3, the one-shot circuit 63 produces rectangular pulses which have a predetermined amplitude and a predetermined time width. The rectangular pulse signal V.sub.4 has a pulse frequency which is the same as the frequency of the sinusoidal wave V.sub.1. Therefore, the value obtained by averaging the rectangular pulse signal V.sub.4 with regard to the time, i.e., a D.C. component of the rectangular pulse signal V.sub.4, changes in proportion to the frequency of the sinusoidal wave V.sub.1 and, hence, changes in proportion to the rotational speed of the rotating body 1. The rectangular pulse signal V.sub.4 is smoothed by the low-pass filter 64. As denoted by V.sub.5 in FIG. 3, therefore, a D.C. voltage signal having a pulsating component is produced at the output terminal of the circuit 6 of FIG. 2, the voltage of the D.C. voltage signal V.sub.5 being proportional to the rotational speed of the rotating body 1.
In the method of detecting the rotational speed by using the apparatus shown in FIGS. 1A, 1B, and 2, the rectangular pulses are smoothed by the low-pass filter 64 to obtain a D.C. wave form. Therefore, the filter must have a relatively large time constant. Further, a limitation is imposed on the response characteristics in detecting the speed, and the ripple ratio increases with a decrease in the rotational speed.